CN117396622A

CN117396622A - Ferritic stainless steel

Info

Publication number: CN117396622A
Application number: CN202280037708.7A
Authority: CN
Inventors: 及川慎司
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2021-06-28
Filing date: 2022-04-14
Publication date: 2024-01-12
Also published as: JP7140309B1; EP4296387A1; US20240247354A1; KR20240000556A; JPWO2023276411A1

Abstract

The invention provides a ferritic stainless steel having good brazability and excellent corrosion resistance after brazing when brazing is performed at high temperature using a Ni-containing brazing filler metal. A ferritic stainless steel having the following composition: contains C in mass%: 0.003 to 0.030 percent, si:0.01 to 1.00 percent of Mn:0.05 to 0.30 percent of P: less than 0.050%, S: less than 0.020%, cr: 24.0-30.0%, ni:1.50 to 3.00 percent of Mo: 1.00-3.00%, al:0.001 to 0.020 percent, nb:0.20 to 0.80 percent of N:0.030% or less, satisfying the following formulas (1) and (2), and the remainder being composed of Fe and unavoidable impurities. Ni-2 (Si+Mn) is more than or equal to 0.00 percent (1) and Cr+1.5Mo+Si+1.5Nb-2.5Ni is less than or equal to 25.0 percent (2) (Ni, si, mn, cr, mo, nb in the formulas (1) and (2) represents the content (mass percent) of each element).

Description

Ferritic stainless steel

Technical Field

The present invention relates to a ferritic stainless steel, and more particularly, to a ferritic stainless steel having excellent brazability and excellent corrosion resistance at the time of brazing at high temperature using a Ni-containing brazing filler metal.

Background

In recent years, from the standpoint of global environmental protection, automobiles are demanded to further improve fuel efficiency and enhance exhaust gas purification. Therefore, the use of heat exchangers for automobiles such as waste heat recoverers and EGR (Exhaust Gas Recirculation) coolers has been expanding.

Here, the exhaust heat recoverer is a device that uses heat of engine cooling water for heating or heats cooling water of an engine using heat of exhaust gas to shorten a heat engine time at the time of engine start, thereby improving fuel efficiency. Typically, the waste heat recoverer is disposed between the catalyst converter and the muffler. The waste heat recoverer is composed of a heat exchanger part formed by combining tubes, plates, fins, side plates and the like, an inlet side tube part and an outlet side tube part. The exhaust gas then enters the heat exchanger section from the inlet side pipe, where the heat is transferred to the cooling water via a heat transfer surface such as fins, and is discharged from the outlet side pipe. In addition, brazing using a Ni-containing brazing filler metal is mainly used for bonding and assembling plates and fins constituting the heat exchanger portion of the waste heat recoverer.

The EGR cooler is composed of a pipe that takes in a part of the exhaust gas from the exhaust manifold or the like, a heat exchanger that cools the taken-in exhaust gas, and a pipe that returns the cooled exhaust gas to the intake side of the engine. As a specific configuration, the EGR cooler has a heat exchanger having both a water flow passage and an exhaust gas passage on a path for returning exhaust gas from the exhaust manifold to the intake side of the engine. By adopting such a structure, the high-temperature exhaust gas on the exhaust side is cooled by the heat exchanger, and the cooled exhaust gas flows back to the intake side of the engine, thereby reducing the combustion temperature of the engine and suppressing NO which is easily generated at high temperatures _X . For reasons such as weight reduction, compactness, and cost reduction, the heat exchanger portion of the EGR cooler is formed by overlapping thin plates in fin form, and brazing using a Ni-containing brazing filler metal is mainly used for bonding and assembling the thin plates.

In this way, since the heat exchanger portions of the exhaust heat recoverer and the EGR cooler are bonded and assembled by brazing using the Ni-containing brazing filler metal, the materials used for these heat exchanger portions are required to have good brazing properties with respect to the Ni-containing brazing filler metal. Further, since the exhaust gas contains nitrogen oxides (NO _X ) Sulfur Oxides (SO) _X ) Hydrocarbons (HC) and thus dew condensation in the heat exchanger becomes highly corrosiveAcid condensed water. Therefore, the materials used in these heat exchanger portions also require corrosion resistance. In particular, since the brazing heat treatment reaches a high temperature, it is necessary to prevent Cr in the grain boundary from reacting with C, N to form Cr carbon nitrogen compound and form a Cr-deficient layer having poor corrosion resistance around the Cr carbon nitrogen compound, so-called sensitization, and to ensure corrosion resistance.

For the above reasons, austenitic stainless steel such as SUS316L or SUS304L is generally used for the heat exchanger portion of the exhaust heat recoverer or the EGR cooler, which is reduced in carbon content to prevent sensitization. However, there are problems in terms of: austenitic stainless steel is costly because of the large amount of Ni contained, and has low fatigue properties in a use environment in which high temperature and intense vibration are constrained due to large thermal expansion, that is, thermal fatigue properties at high temperature.

Therefore, steel other than austenitic stainless steel has been studied for use in heat exchanger portions of waste heat recoverers and EGR coolers.

For example, patent document 1 discloses a ferritic stainless steel which is made of a material for a waste heat recoverer or an EGR cooler, and which is made of a material that is brazed to form an oxide film containing 16.0% or more of Nb in terms of cation fraction, thereby securing corrosion resistance.

Patent document 2 discloses a ferritic stainless steel in which corrosion resistance is ensured by controlling the addition amounts of Al, ti, and Si as a material for a waste heat recoverer and an EGR cooler.

Patent document 3 discloses a ferritic stainless steel which ensures corrosion resistance by controlling the contents of Cr, si, and Al in an oxide film after brazing, and the film thickness of the oxide film, as a material for heat exchangers and fuel supply components.

Patent document 4 discloses a ferritic stainless steel in which components such as Cr, cu, al, ti are added in a predetermined relation and the addition amount of Al and Ti is suppressed to ensure brazeability as a material for an EGR cooler.

Further, patent document 5 discloses, as an EGR cooler member having a structure joined by Ni brazing, a ferritic stainless steel in which brazing property is ensured by suppressing the addition amount of Al, ti, zr.

Further, patent document 6 discloses a ferritic stainless steel for brazing, which ensures brazeability by suppressing the addition amount of Ti and Zr.

Prior art literature

Patent literature

Patent document 1: japanese patent No. 6157664

Patent document 2: japanese patent No. 6159775

Patent document 3: japanese patent No. 6270821

Patent document 4: japanese patent application laid-open No. 2010-121208

Patent document 5: japanese patent laid-open No. 2009-174040

Patent document 6: japanese patent laid-open No. 2010-285683

Disclosure of Invention

However, in the techniques described in patent documents 1to 6, there are cases where the brazability is insufficient depending on the brazing filler metal used, the brazing conditions, and the like. In particular, in the conventional art, it cannot be said that the excellent corrosion resistance as described above can be obtained and the solderability at high temperature using the Ni-containing brazing filler metal can be sufficiently ensured.

The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a ferritic stainless steel having excellent brazability and excellent corrosion resistance after brazing, when brazing is performed at a high temperature using a Ni-containing brazing filler metal.

In the present specification, good brazability means that after a steel sheet having a Ni-containing brazing material (JIS standard: BNi-5) applied to the surface thereof is subjected to a brazing treatment in which the steel sheet is heated at 1170 ℃ in a nitrogen carrier gas atmosphere of 1Torr for 10 minutes and cooled to room temperature, the ratio of the equivalent circle diameter of the brazing material after heating to the equivalent circle diameter of the brazing material before heating (the spreading rate of the brazing material) is 150% or more.

The excellent corrosion resistance means the following conditions: using the steel sheet after the brazing treatment with the Ni-containing brazing filler metal, a test piece of 20mm square was collected from the portion where the brazing filler metal was not adhered, the measurement surface of 11mm square was left, and the test piece was covered with a sealing material, and further immersed in a 3.5% aqueous NaCl solution at 30℃to have a pitting corrosion potential Vc'100 of 300mV (vs SCE) or more measured in accordance with JIS G0577, except for the concentration of the aqueous NaCl solution.

The inventors of the present invention have conducted intensive studies on the relationship between the constituent elements of various stainless steels and brazability when brazing is performed at high temperature using a Ni-containing brazing filler metal.

As a result, it was found that the wettability with the Ni-containing brazing filler metal was improved by suppressing the Al content in the stainless steel, adding an appropriate amount of Ni to the stainless steel, and further appropriately suppressing the Si and Mn contents with respect to the Ni content.

Further, the relationship between the constituent elements of various stainless steels and the corrosion resistance after brazing has been studied intensively. As a result, it was found that the sigma phase (σ phase) precipitated and the corrosion resistance was reduced by the high-temperature heat treatment of brazing. Here, the σ phase is an intermetallic compound containing a large amount of Cr and Mo, and a Cr and Mo deficiency layer is formed around the intermetallic compound, so that corrosion resistance after brazing is reduced. Further, as a result of the study, it was found that the precipitation of sigma phase in the high-temperature heat treatment of brazing can be suppressed by containing a proper amount of Ni and further appropriately suppressing the contents of Cr, mo, si and Nb with respect to the Ni content.

The present invention has been completed based on the above-described findings and through further studies. That is, the gist of the present invention is as follows.

[1] A ferritic stainless steel having the following composition:

the alloy comprises the following components in percentage by mass:

C：0.003～0.030％、

Si：0.01～1.00％、

Mn：0.05～0.30％、

p:0.050% or less,

S: less than 0.020%,

Cr：24.0～30.0％、

Ni：1.50～3.00％、

Mo：1.00～3.00％、

Al：0.001～0.020％、

Nb：0.20～0.80％、

N: the content of the catalyst is less than or equal to 0.030 percent,

the following formulas (1), (2) are satisfied, and the remainder is composed of Fe and unavoidable impurities.

Ni－2(Si+Mn)≥0.00％···(1)

Cr+1.5Mo+Si+1.5Nb－2.5Ni≤25.0％···(2)

(Ni, si, mn, cr, mo, nb in the formulae (1) and (2) represents the content (mass%) of each element.)

[2] The ferritic stainless steel according to the above [1], wherein the ferritic stainless steel further comprises, in mass%, a metal selected from the group consisting of:

Cu：0.01～1.00％、

Co：0.01～1.00％、

W：0.01～2.00％

1 or more than 2 of them.

[3] The ferritic stainless steel according to [1] or [2], wherein the ferritic stainless steel further comprises, in mass%, a metal selected from the group consisting of:

Ti：0.01～0.10％、

V：0.01～0.20％、

Zr：0.01～0.10％、

Mg：0.0005～0.0050％、

Ca：0.0005～0.0050％、

B：0.0005～0.0050％、

REM (rare earth metal): 0.001 to 0.100 percent,

Sn：0.001～0.100％、

Sb：0.001～0.100％

1 or more than 2 of them.

[4] The ferritic stainless steel according to [1] or [2], which is used for a waste heat recoverer or an exhaust gas recirculation device of one or more joints assembled by brazing.

[5] The ferritic stainless steel according to [3], which is used for a waste heat recoverer or an exhaust gas recirculation device in which one or more joints are assembled by brazing.

According to the present invention, it is possible to provide a ferritic stainless steel having good brazability and excellent corrosion resistance after brazing when brazing is performed at high temperature using a Ni-containing brazing filler metal.

Detailed Description

The present invention will be specifically described below.

First, the reason why the composition of the steel is limited to the above-described range in the present invention will be described. The unit of the content of the element in the composition of the steel is "mass%", but is indicated by "%" unless otherwise specified below.

C：0.003～0.030％

The strength is improved with a larger C content, and the workability is improved with a smaller C content. Here, in order to obtain sufficient strength, it is necessary to contain 0.003% or more of C. However, if the C content exceeds 0.030%, the workability becomes remarkable, and Cr carbide precipitates in the grain boundaries to cause sensitization, and the corrosion resistance is lowered. Therefore, the C content is in the range of 0.003 to 0.030%. The C content is preferably 0.004% or more. The C content is preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.010% or less.

Si：0.01～1.00％

Si is an element used as a deoxidizing material. This effect is obtained when Si is contained in an amount of 0.01% or more. However, if the Si content exceeds 1.00%, si concentrates such as Si oxides and Si nitrides are formed on the surface of the steel sheet during the brazing heat treatment, and the brazability is lowered. In addition, in the brazing process at high temperature using the Ni-containing brazing filler metal, the sigma phase precipitates, and the corrosion resistance is lowered. Therefore, the Si content is in the range of 0.01 to 1.00%. The Si content is preferably 0.50% or more, more preferably 0.60% or more, and still more preferably 0.70% or more. The Si content is preferably 0.85% or less, more preferably 0.80% or less.

Mn：0.05～0.30％

Mn has deoxidizing effect, and the effect is obtained when Mn is contained at 0.05% or more. However, if the Mn content exceeds 0.30%, mn concentrates are formed on the surface of the steel sheet during the brazing heat treatment, and the brazability is lowered. Therefore, the Mn content is in the range of 0.05 to 0.30%. The Mn content is preferably 0.10% or more. The Mn content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.

P: less than 0.050%

P is an element inevitably contained in steel, and excessive content is liable to cause grain boundary corrosion. Its tendency becomes remarkable when it contains more than 0.050% of P. Therefore, the P content is 0.050% or less. The P content is preferably 0.040% or less, more preferably 0.030% or less. The lower limit of the P content is not particularly limited. However, excessive P removal leads to an increase in cost, so that the P content is preferably 0.005% or more.

S: less than 0.020%

S is an element inevitably contained in steel, and when S is contained in an amount exceeding 0.020%, precipitation of MnS is promoted, and corrosion resistance is lowered. Therefore, the S content is 0.020% or less. The S content is preferably 0.010% or less. The lower limit of the S content is not particularly limited. However, excessive S removal leads to an increase in cost, so that the S content is preferably 0.0005% or more.

Cr：24.0～30.0％

Cr is an important element for ensuring corrosion resistance of stainless steel. When the Cr content is less than 24.0%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content exceeds 30.0%, the σ phase precipitates during brazing, and the corrosion resistance decreases. Therefore, the Cr content is in the range of 24.0 to 30.0%. The Cr content is preferably 24.5% or more, more preferably 25.0% or more. The Cr content is preferably 28.0% or less, more preferably 26.5% or less.

Ni：1.50～3.00％

Ni is one of the important elements in the present invention. By containing 1.50% or more of Ni, the brazability with a Ni-containing solder is improved. The mechanism of improving the Ni solderability by the inclusion of Ni is not clear, but it is considered that when an appropriate amount of Ni is included in the base material, the wettability is improved by the interaction with Ni included in the brazing filler metal. In addition, sigma phase precipitation during brazing can be suppressed. However, if the Ni content exceeds 3.00%, stress corrosion cracking sensitivity becomes high. Therefore, the Ni content is in the range of 1.50 to 3.00%. The Ni content is preferably 1.75% or more, more preferably 2.00% or more. The Ni content is preferably 2.75% or less, more preferably 2.50% or less.

Mo：1.00～3.00％

Mo stabilizes the passivation film of stainless steel to improve corrosion resistance. This effect is obtained when the Mo content is 1.00% or more. However, if the Mo content exceeds 3.00%, σ phase precipitates during brazing, and the corrosion resistance decreases. Therefore, the Mo content is in the range of 1.00 to 3.00%. The Mo content is preferably 1.25% or more, more preferably 1.50% or more. The Mo content is preferably 2.50% or less, more preferably 2.00% or less.

Al：0.001～0.020％

Al is an element useful for deoxidization, and the effect is obtained when Al is contained at 0.001% or more. However, if the Al content exceeds 0.020%, al concentrates such as Al oxide and Al nitride are formed on the surface of the steel during the brazing process, and the wettability and adhesion of the brazing filler metal are reduced, making brazing difficult. Therefore, the Al content is in the range of 0.001 to 0.020%. The Al content is preferably 0.015% or less.

Nb：0.20～0.80％

Nb is an element that suppresses the decrease in corrosion resistance (sensitization) due to precipitation of Cr carbon nitrogen compounds by bonding with C and N. This effect is obtained when the Nb content is 0.20% or more. On the other hand, if the Nb content exceeds 0.80%, σ phase precipitates during brazing, and the corrosion resistance decreases. Accordingly, the Nb content is in the range of 0.20 to 0.80%. The Nb content is preferably 0.25% or more, more preferably 0.30% or more. The Nb content is preferably 0.60% or less, more preferably 0.35% or less.

N: less than 0.030 percent

If the N content exceeds 0.030%, corrosion resistance and workability are reduced. Therefore, the N content is 0.030% or less. The N content is preferably 0.025% or less. Further preferably, the N content is 0.020% or less. The lower limit of the N content is not particularly limited, but excessive reduction of the N content leads to an increase in cost, so that the N content is preferably 0.003% or more.

Ni－2(Si+Mn)≥0.00％···(1)

Ni, si, mn in the formula (1) represent the content (mass%) of each element.

In the present invention, ni, si and Mn are each set to a predetermined content in order to improve the brazability. Further, the inventors have conducted intensive studies and found that when Ni-2 (Si+Mn) (Ni content minus 2 times the total of Si content and Mn content) is less than 0.00%, desired brazability cannot be obtained. The reason for this is considered that Ni improves brazeability, but Si and Mn inhibit brazeability, and therefore the balance of these elements has a large influence on brazeability. Therefore, in the present invention, ni-2 (Si+Mn) is set to 0.00% or more while Ni content, si content and Mn content are set to the above ranges, respectively. Ni-2 (Si+Mn) is preferably 0.50% or more. In particular, a more excellent brazability can be obtained by setting the Cr content to less than 26.0%, the Al content to 0.015% or less, and the Ni-2 (Si+Mn) content to 0.50% or more.

Cr+1.5Mo+Si+1.5Nb－2.5Ni≤25.0％···(2)

Cr, mo, si, nb and Ni in the formula (2) represent the content (mass%) of each element.

In the present invention, cr, mo, si, nb and Ni are each contained in a predetermined amount in order to suppress precipitation of sigma phase during brazing. Further, the present inventors have made intensive studies and found that if cr+1.5mo+si+1.5nb-2.5Ni is greater than 25.0%, sigma phase precipitates during brazing and corrosion resistance is lowered. The reason for this is considered that Ni suppresses precipitation of sigma phase, but Cr, mo, si, and Nb promote precipitation of sigma phase, so the balance of these elements has a large influence on precipitation of sigma phase. Therefore, in the present invention, the content of Cr, mo, si, nb and Ni is set to be 25.0% or less, respectively, while the content of Cr+1.5Mo+Si+1.5Nb-2.5Ni is set to be the above range.

The basic components (essential components) of the ferritic stainless steel of the present invention are described above. In the composition of the components of the present invention, the components (the remainder) other than the above are Fe and unavoidable impurities.

The ferritic stainless steel of the present invention may further contain 1 or 2 or more kinds selected from Cu, co, and W in the following ranges, respectively.

Cu：0.01～1.00％

Cu is an element that improves corrosion resistance. This effect is obtained when the Cu content is 0.01% or more. However, if the Cu content exceeds 1.00%, hot workability is lowered. Therefore, when Cu is contained, the Cu content is in the range of 0.01 to 1.00%. When Cu is contained, the Cu content is more preferably 0.10% or more. When Cu is contained, the Cu content is more preferably 0.80% or less, and still more preferably 0.60% or less.

Co：0.01～1.00％

Co is an element that improves corrosion resistance. This effect is obtained when the Co content is 0.01% or more. However, if the Co content exceeds 1.00%, the workability is lowered. Therefore, when Co is contained, the Co content is in the range of 0.01 to 1.00%. When Co is contained, the Co content is more preferably 0.05% or more. When Co is contained, the Co content is more preferably 0.70% or less.

W：0.01～2.00％

W is an element for improving corrosion resistance. This effect is obtained when the W content is 0.01% or more. However, if the W content exceeds 2.00%, the σ phase precipitates during brazing. Therefore, when W is contained, the W content is in the range of 0.01 to 2.00%. When W is contained, the W content is more preferably 0.05% or more. When W is contained, the W content is more preferably 1.00% or less.

The ferritic stainless steel of the present invention may further contain 1 or 2 or more kinds selected from Ti, V, zr, mg, ca, B, REM, sn, sb in the following ranges, respectively.

Ti：0.01～0.10％

Ti has an effect of bonding with C and N contained in steel to prevent sensitization. This effect is obtained when Ti is contained in an amount of 0.01% or more. On the other hand, ti is an element active on oxygen, and when it is contained in an amount exceeding 0.10%, a Ti oxide film is formed on the surface of steel during the brazing process, and the brazability is lowered. Therefore, when Ti is contained, the Ti content is in the range of 0.01 to 0.10%. When Ti is contained, the Ti content is more preferably 0.05% or less.

V：0.01～0.20％

Like Ti, V is bonded to C and N contained in steel to prevent sensitization. These effects are obtained when the V content is 0.01% or more. On the other hand, if the V content exceeds 0.20%, the workability is lowered. Therefore, when V is contained, the V content is in the range of 0.01 to 0.20%. When V is contained, the V content is more preferably 0.15% or less, and still more preferably 0.10% or less.

Zr：0.01～0.10％

Like Ti and Nb, zr is an element that binds to C and N contained in steel and suppresses sensitization. This effect is obtained when the Zr content is 0.01% or more. On the other hand, if the Zr content exceeds 0.10%, the workability is lowered. Accordingly, when Zr is contained, the Zr content is in the range of 0.01 to 0.10%. When Zr is contained, the Zr content is more preferably 0.03% or more. When Zr is contained, the Zr content is more preferably 0.05% or less.

Mg：0.0005～0.0050％

Mg acts as a deoxidizer. This effect is obtained when the Mg content is 0.0005% or more. However, if the Mg content exceeds 0.0050%, the toughness of the steel decreases and the manufacturability decreases. Therefore, when Mg is contained, the Mg content is in the range of 0.0005 to 0.0050%. When Mg is contained, the Mg content is more preferably 0.0020% or less.

Ca：0.0005～0.0050％

Ca improves the penetration of the welded portion to make the welded portion weldable. This effect is obtained when the Ca content is 0.0005% or more. However, if the Ca content exceeds 0.0050%, caS is formed by bonding with S, and corrosion resistance is lowered. Therefore, when Ca is contained, the Ca content is in the range of 0.0005 to 0.0050%. When Ca is contained, the Ca content is more preferably 0.0010% or more. In addition, when Ca is contained, the Ca content is more preferably 0.0040% or less.

B：0.0005～0.0050％

B is an element for improving brittleness in secondary processing. This effect is obtained when the B content is 0.0005% or more. However, if the B content exceeds 0.0050%, ductility decreases due to solid solution strengthening. Therefore, when B is contained, the B content is in the range of 0.0005 to 0.0050%.

REM (rare earth metal): 0.001 to 0.100 percent

REM (rare earth metal: elements having atomic numbers 57 to 71 such as La, ce, nd, etc.) is an element effective for deoxidation. This effect is obtained when the REM content is 0.001% or more. However, if the REM content exceeds 0.100%, hot workability is lowered. Therefore, when REM is contained, the REM content is in the range of 0.001 to 0.100%. When REM is contained, the REM content is more preferably 0.010% or more. When REM is contained, the REM content is more preferably 0.050% or less. REM is a generic term for 15 elements, i.e., sc, Y and lutetium (Lu) from lanthanum (La) having atomic number 57 to lutetium (Lu) having atomic number 71, and the REM content is the total content of these elements.

Sn：0.001～0.100％

Sn is an element effective for suppressing surface roughness during processing. This effect is obtained when the Sn content is 0.001% or more. However, if the Sn content exceeds 0.100%, hot workability is lowered. Therefore, when Sn is contained, the Sn content is in the range of 0.001 to 0.100%. When Sn is contained, the Sn content is more preferably 0.050% or less.

Sb：0.001～0.100％

Like Sn, sb is an element effective for suppressing surface roughness during processing. This effect is obtained when the Sb content is 0.001% or more. However, if the Sb content exceeds 0.100%, the workability is lowered. Therefore, when Sb is contained, the Sb content is in the range of 0.001 to 0.100%. When Sb is contained, the Sb content is more preferably 0.050% or less.

Next, a preferred method for producing the ferritic stainless steel of the present invention will be described.

The method for producing the ferritic stainless steel of the present invention is not particularly limited, and for example, steel having the above-described composition of the present invention can be produced by melting steel in a melting furnace such as a converter or an electric furnace, or further by secondary refining such as ladle refining or vacuum refining. Then, a steel sheet (billet) is produced by a continuous casting method or an ingot-cogging rolling method, and the billet is hot-rolled to produce a hot-rolled sheet, or if necessary, the hot-rolled sheet is subjected to hot-rolled sheet annealing to produce a hot-rolled annealed sheet. Then, the hot-rolled sheet or the hot-rolled annealed sheet is cold-rolled to obtain a cold-rolled sheet having a desired sheet thickness, or the cold-rolled sheet may be further cold-rolled sheet annealed as necessary to obtain a cold-rolled annealed sheet.

The conditions for hot rolling, cold rolling, hot-rolled sheet annealing, cold-rolled sheet annealing, and the like are not particularly limited, and may be conventional methods.

In the steelmaking step of steelmaking, it is preferable that the steel melted in a converter, an electric furnace, or the like is secondarily refined by a VOD (Vacuum Oxygen Decarburization (vacuum deoxidation)) method or the like to produce steel containing the above-mentioned essential components and components added as needed. The molten steel to be melted can be formed into a steel slab (billet) by a known method, but from the viewpoint of productivity and quality, a continuous casting method is preferably used. The steel slab is then preferably heated to 1050 to 1250 ℃ and hot rolled to produce a hot rolled sheet of a desired sheet thickness. Of course, the sheet may be heat-processed other than by heat processing. The hot-rolled sheet is preferably continuously annealed at 900 to 1150 ℃ as needed, and then descaled by pickling or the like to obtain a hot-rolled product. The oxide scale may be removed by shot blasting, brushing, or the like before pickling, if necessary.

Further, the hot-rolled product (hot-rolled annealed sheet or the like) may be formed into a cold-rolled product through a process such as cold rolling. In this case, the cold rolling may be performed 1 time, but from the viewpoint of productivity and quality requirements, the cold rolling may be performed 2 or more times with intermediate annealing interposed therebetween. The total reduction of the cold rolling for 1 or 2 times or more is preferably 60% or more, more preferably 70% or more. The cold-rolled steel sheet is then preferably subjected to continuous annealing (finish annealing) at a temperature of preferably 900 to 1150 ℃ and more preferably 950 to 1150 ℃ and pickled to obtain a cold-rolled product. The continuous annealing may be performed by bright annealing, and pickling may be omitted. Further, the shape, surface roughness, and material of the steel sheet may be adjusted by performing skin pass rolling or the like after finish annealing according to the purpose.

The ferritic stainless steel of the present invention described above is suitably used for a waste heat recoverer and an exhaust gas recirculation device in which one or more joints are assembled by brazing. The heat exchanger element is particularly suitable for use in the waste heat recoverer and the exhaust gas recirculation device.

Examples

Steel having the composition shown in table 1 was melted in a vacuum melting furnace, heated at 1150 ℃ for 1 hour, and hot-rolled into a hot-rolled sheet having a thickness of 4.0 mm. After the hot rolled sheet was annealed at 1080℃for 1 minute, the surface was polished by grinding to remove scale, and cold rolled to a sheet thickness of 1.0mm. The cold-rolled annealed sheet was subjected to finish annealing in an ammonia-decomposing gas atmosphere at 1040 ℃ for 1 minute, and the surface thereof was polished to 600 mesh with a silicon carbide polishing paper, and subjected to degreasing with acetone, and subjected to a test.

The cold-rolled annealed sheet was brazed using a Ni-containing brazing filler metal as described below, and the results of (1) evaluation of brazability, and (2) measurement of the amount of sigma phase deposition and (3) evaluation of corrosion resistance were performed on the cold-rolled annealed sheet after the brazing treatment are shown in table 2.

(1) Evaluation of brazeability

Test pieces 50mm in width and 50mm in length were cut from the cold-rolled annealed sheet thus produced, and subjected to the following brazing treatment: a Ni-containing solder (JIS standard: BNi-5) having a diameter of 10mm and a thickness of 1mm was applied to the surface of a horizontal test piece, and then the test piece coated with the Ni-containing solder was heated in a nitrogen carrier gas atmosphere of 1170 ℃ and 1Torr with the solder-coated surface facing upward and placed horizontally, and then cooled to room temperature. Then, the equivalent circle diameter of the solder on the surface of the test piece (equivalent circle diameter of the solder after heating) was measured. Then, the ratio (spreading rate of the solder) of the equivalent circle diameter of the solder after heating to the diameter of the solder before heating (10 mm, equivalent circle diameter is also the same) was determined, and the evaluation was performed according to the following criteria.

Spreading ratio of solder after heating with respect to solder before heating= (equivalent circle diameter of solder after heating/diameter of solder before heating (10 mm)) ×100 (%)

Good (acceptable, particularly good): 160% or more

O (pass): 150% or more and less than 160%

X (reject): less than 150%

(2) Measurement of sigma phase precipitation amount

Using test pieces of each cold-rolled annealed sheet after the brazing treatment, a sample for observation of L cross section was collected from a portion where no brazing filler metal was adhered, and after electrolytic polishing and aqua regia etching, the amount of sigma phase deposition (area%) was measured by a dot count method according to ASTM E562 in a view field of 500 times observation with an optical microscope. The observation position is the center portion of the thickness of the sample.

O: less than 1.0%

X: more than 1.0%

(3) Evaluation of Corrosion resistance

Using the test piece of each cold-rolled annealed sheet after the brazing treatment, a 20mm square test piece was taken from a portion to which no brazing filler metal was adhered, and a measurement surface of 11mm square was left on the test piece, and the test piece was covered with a silicone resin sealing material. Next, the test piece was immersed in a 3.5% aqueous NaCl solution at 30℃to measure the pitting potential according to JIS G0577, except for the concentration of the aqueous NaCl solution. After 10 minutes of holding at natural potential, the current density was increased to 100. Mu.A/cm by potentiodynamic method with potential sweep rate of 20mV/min ² The potential at this time was referred to as cavitation potential Vc'100, and the values thereof are shown in Table 2. In consideration of the conditions under which the heat exchanger portions of the exhaust heat recoverer and the EGR cooler are used, it can be determined that the corrosion resistance is excellent if the pitting corrosion potential Vc'100 is 300mV (vs SCE) or more, which corresponds to SUS304L that is well recorded in the heat exchanger portions of the exhaust heat recoverer and the EGR cooler.

O (pass): 300mV (vs SCE) or more

X (reject): less than 300mV (vs SCE) less than

TABLE 1

The other part than the above component composition is Fe and unavoidable impurities

Underline is outside the scope of the present invention

Table 2:

as is clear from Table 2, in each of invention examples No. 1to 27, good brazability and excellent corrosion resistance were obtained. In particular, no.6, 9, 10, 12, 13, 16, 18, 19 and 21, which have a Cr content of less than 26.0%, an Al content of 0.015% or less and a Ni-2 (Si+Mn). Gtoreq.0.50%, show particularly good brazability.

In contrast, in comparative examples nos. 28 to 38, in which the composition was outside the proper range, good brazability and excellent corrosion resistance could not be simultaneously satisfied.

More specifically, in comparative example No.28 (steel mark B1), since the Cr content exceeds the upper limit value of the present invention, the σ phase is precipitated in the microstructure after brazing by more than 1.0%, and excellent corrosion resistance cannot be obtained.

In comparative example No.29 (steel mark B2), since the Mo content exceeds the upper limit value of the present invention, the sigma phase is precipitated in the microstructure after brazing exceeding 1.0%, and excellent corrosion resistance cannot be obtained.

In comparative example No.30 (steel mark B3), since the Al content exceeds the upper limit value of the present invention, good brazability cannot be obtained.

In comparative example No.31 (steel mark B4), since the Si content exceeds the upper limit value of the present invention, good brazability cannot be obtained. In addition, the σ phase precipitated in the microstructure after brazing exceeds 1.0%, and excellent corrosion resistance cannot be obtained.

In comparative example No.32 (steel mark B5), since the Mn content exceeds the upper limit value of the present invention, good brazability cannot be obtained. In addition, excellent corrosion resistance cannot be obtained.

In comparative example No.33 (steel mark B6), since the Nb content exceeds the upper limit value of the present invention, the sigma phase is precipitated in excess of 1.0% in the microstructure after brazing, and excellent corrosion resistance cannot be obtained.

In comparative example No.34 (steel mark B7), since the Cr content was less than the lower limit value of the present invention, excellent corrosion resistance could not be obtained.

In comparative example No.35 (steel mark B8), since the Mo content is less than the lower limit value of the present invention, excellent corrosion resistance could not be obtained.

In comparative example No.36 (steel mark B9), since the Ni content was less than the lower limit value of the present invention, good brazability could not be obtained. Further, the σ phase precipitated in the microstructure after brazing was more than 1.0%, and excellent corrosion resistance could not be obtained.

In comparative example No.37 (steel mark B10), all the components were within the predetermined range, but the formula (2) was not satisfied, and the sigma phase precipitated more than 1.0%, and excellent corrosion resistance could not be obtained.

In comparative example No.38 (steel mark B11), all the components were within a predetermined range, but the formula (1) was not satisfied, and good brazability could not be obtained.

Industrial applicability

According to the present invention, a ferritic stainless steel suitable for use in an exhaust gas recirculation device such as a waste heat recoverer and a heat exchanger member of an EGR cooler assembled by brazing can be obtained, and therefore, the present invention is industrially extremely useful.

Claims

1. A ferritic stainless steel having the following composition:

the alloy comprises the following components in percentage by mass:

C：0.003～0.030％、

Si：0.01～1.00％、

Mn：0.05～0.30％、

p:0.050% or less,

S: less than 0.020%,

Cr：24.0～30.0％、

Ni：1.50～3.00％、

Mo：1.00～3.00％、

Al：0.001～0.020％、

Nb：0.20～0.80％、

N: the content of the catalyst is less than or equal to 0.030 percent,

the following formulas (1) and (2) are satisfied, and the balance is made up of Fe and unavoidable impurities;

Ni－2(Si+Mn)≥0.00％···(1)

Cr+1.5Mo+Si+1.5Nb－2.5Ni≤25.0％···(2)

ni, si, mn, cr, mo, nb in the formulas (1) and (2) represents the content of each element in mass%.

2. The ferritic stainless steel according to claim 1, further comprising, in mass%, a metal selected from the group consisting of:

Cu：0.01～1.00％、

Co：0.01～1.00％、

W：0.01～2.00％

1 or more than 2 of them.

3. The ferritic stainless steel according to claim 1 or 2, further comprising, in mass%, a metal selected from the group consisting of:

Ti：0.01～0.10％、

V：0.01～0.20％、

Zr：0.01～0.10％、

Mg：0.0005～0.0050％、

Ca：0.0005～0.0050％、

B：0.0005～0.0050％、

REM is a rare earth metal: 0.001 to 0.100 percent,

Sn：0.001～0.100％、

Sb：0.001～0.100％

1 or more than 2 of them.

4. The ferritic stainless steel according to claim 1 or 2, being used for waste heat recoverer or exhaust gas recirculation device of more than one joint assembled by brazing.

5. A ferritic stainless steel according to claim 3, for use in waste heat recoverers or exhaust gas recirculation devices for assembling more than one joint by brazing.